Interaction of Metal Cations with Functionalised Hydrocarbons in the Gas Phase: Further Experimental Evidence for Solvation of Metal Ions by the Hydrocarbon Chain

Relative affinity measurements of monovalent metal ions (=Li⁺, Cu⁺ and Ag⁺) towards aliphatic amines, alcohols and methyl alkanoates (P) have been performed using the kinetic method on the dissociation of metal-bound dimer ions of the type P₁–M⁺–P₂. It was found that the cations' affinity towards long chain (≥C₄ chain length) n- and s-alkylamines, n-alkanols and methyl n-alkanoates was unexpectedly enhanced. This is attributed to a bidentate interaction of the metal ion with the amine, alcohol or ester functional group and the aliphatic chain, paralleling earlier observations on metal-bound nitriles. Methyl substitution at the functional group (s-alkylamines compared with n-alkylamines) serves to strengthen only the N M⁺ bond, and this can be rationalised by the larger proton affinities of s-alkylamines compared to n-alkylamines. This substitution, however, has no effect on the metal ion–hydrocarbon bond. In contrast, methyl substitution remote from the functional group, as in iso-pentylamine, does lead to strengthening of the metal ion–hydrocarbon bond. The cuprous ion affinity of hexadecylamine, C₁₆H₃₃NH₂, was found to be as large as that for ethylenediamine (352 kJ mol⁻¹), known to be a strong copper binding agent. It is argued that such a metal ion - hydrocarbon interaction does not occur in the metal bound dimers.